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"At Qantas safety is our first priority and a number of flights have been cancelled or re-routed to avoid the volcanic ash cloud."

It added that its morning services from Sydney to Bangkok, London, Singapore and Frankfurt would fly, but international flights after 3pm were under review.

Passengers waiting to travel from Adelaide were understanding about the delay.

Darren Martin said he was prepared to wait until the skies had been declared safe to fly in.

"We knew it was floating about ... we found out this morning we're stuck here till Thursday," he told the Adelaide Advertiser.

"I'd rather not fly if it's too hazardous, I'd rather fly safer."

Meteorologists said that while the ash had thinned during its travels around the world it was still clearly visible on satellite images and was travelling at an altitude of 5-8 miles, generally cruising level for aircraft.

The cloud first entered Australian and New Zealand airspace just over a week ago, causing some airlines to ground 700 flights from Perth to New Zealand while others chose to divert their planes under and around the plume.

Ash poses a significant threat to aircraft because once sucked into engines it can be converted into molten glass as a result of the high temperatures and potentially cause an engine to fail.